Fix problems with Python, improve installation, and improve tests

[rafaelab]

This PR supersedes #457.
It mainly improves the handling of python, but it also contains many other improvements/fixes:

• Use CMake's built-in Python finder and remove CRPropa's Python finder.
• Remove all references to Python2 and completely drops support
• Fix a bug common to several tests when using clang (gtest cannot correctly trace exceptions leading to failures when calling EXPECT_THROW)
• Fix a bug in testInteraction related to EMPairProduction, in OS X.
• Possible solution the long-standing issue related to the magnetic lenses and python
• Minor improvements in workflow
• Apply some code style convention to *.i files and CMake
• Improve code style in tests

One of the OSX tests (testPythonExtension.py) is expected to fail because of a problem applying NumPy functions to Vector3d in one of the tests (testArrayInterface). This does not happen, for example, in Ubuntu.
Even if this particular test fails, at least it is known and doesn't seem to affect CRPropa in any significant way.

[rafaelab]

Thanks for the review, @JulienDoerner
I think I've addressed most of the issues you've raised so far and after a long day of attempts I managed to get crpropa-example-test to run, to some extent.

I tried to make the installation default to Python_SITE_PACKAGES (actually, Python_SITELIB) only if the user is not trying to do a local installation using CMAKE_INSTALL_PREFIX. This way, we don't affect the previous behaviour too much, and people like myself who prefer to choose the installation paths are satisfied. What do you think?

I've made one further change: I commented out the test that was leading to the problem in the OSX tests. Unless you have any objections, I'd suggest we keep it that way until we figure out how to solve the problem. And it is better not to delete the corresponding lines yet, since they might be useful in the future.

[lukasmerten]

It's more or less working for me.

However, I would also prefer an option to specify the CMAKE_INSTALL_PREFIX and Python_INSTALL_PACKAGE_DIR independently of each other. I need to set the CMAKE_INSTALL_PREFIX to something, which is different from the default /usr/local as we do not have write access to that location on our university machines. This leads in the current PR to installing the python module in CMAKE_INSTALL_PREFIX/crpropa which would require manually adapting the python PATH for my virtual envs...

EDIT:
The tests, however, are all passing for me. So the lensing bug seems to be fixed with this PR.

[JulienDoerner]

Hey @rafaelab,

I think adding this variable Python_INSTALL_PACKAGE_DIR is a good idea. But for me I could not change (or see) it using ccmake. It only appers in the printed log.

And also this does not reproduce the old installation behaviour. In this case (given PREFIX) all installation parts are stored on the same level (python package PREFIX/crpropa, shared data PREFIX/share/crpropa, lib PREFIX/lib/libcrpropa.so and include PREFIX\include\CRPropa.h).
I would think the best way would be to allow for the user defined path but default it to PREFIX/lib/python3.X/crpropa. In this case the installation procedure would stay the same as in our installation documentation.

[rafaelab]

Thanks for the feedback, @JulienDoerner and @lukasmerten.
Now the flag Python_INSTALL_PACKAGE_DIR is completely independent from CMAKE_INSTALL_PREFIX.

The remaining issue is what @JulienDoerner said about $PREFIX/lib/python3.X/crpropa. This is the internal structure of python packages. But in this case, the user explicitly wants to control all paths, so I fail to see a reason to keep it this way. Why add so many unnecessary layers when we can just have $Python_INSTALL_PACKAGE_DIR directly (at the level of $PREFIX or perhaps $Python_INSTALL_PACKAGE_DIR/python?

[JulienDoerner]

hey @rafaelab,

I think the change is good, as it is. My point for the python3.X/site-packages/ was only about the default value for the $Python_INSTALL_PACKAGE_DIR. But I agree that installing it directly in this level is the best solution.
And defaulting it into the Python_SITELIB will keep the previous installation behaviour.

[lukasmerten]

Thanks for your effort @rafaelab.
Seems to work for me and I will approve this later.

Just for my records:
You can set the flag for the python installation path with
($~/build) cmake -DPython_INSTALL_PACKAGE_DIR=..

Just for curiosity:
Is there a way to add Python_INSTALL_PACKAGE_DIR to the parameters that are listed when I run ccmake?

[rafaelab]

Just for my records: You can set the flag for the python installation path with ($~/build) cmake -DPython_INSTALL_PACKAGE_DIR=..

Just for curiosity: Is there a way to add Python_INSTALL_PACKAGE_DIR to the parameters that are listed when I run ccmake?

No idea, @lukasmerten. I did add a status message with the value of this flag, but I haven't invested too much time in adding it to the parameter. I thought it would show up there automatically.
In any case, I will keep this on my to-do list for next time I make general changes to CRPropa.

[potassium-chloride]

Something goes wrong on this line. See issue #448 . Please, check behavior of this code on NumPy version 1.21.6.